wrap uevent-watch in a service

modules/cdc-ncm/default.nix

@@ -4,6 +4,10 @@ let
   inherit (lib) mkOption types;
   svc = config.system.service;
 in {
+  imports = [
+    ../service-trigger
+  ];
+
   options = {
     system.service.wwan = mkOption {
       type = liminix.lib.types.serviceDefn;

modules/cdc-ncm/wwan.nix

@@ -53,11 +53,16 @@ let
         buildInputs = [ modeswitch ];
         run = "${uevent-watch}/bin/uevent-watch -s ${modeswitch.name} devtype=usb_device product=12d1/14fe/102";
       })
-      (longrun {
-        name = "watch-for-modem";
-        isTrigger = true;
-        buildInputs = [ atz ];
-        run = "${uevent-watch}/bin/uevent-watch -n /dev/modem -s ${atz.name} subsystem=tty attrs.idVendor=12d1 attrs.idProduct=1506";
+      (svc.uevent-rule.build {
+        service = atz;
+        terms = {
+          subsystem = "tty";
+          attrs = {
+            idVendor = "12d1";
+            idProduct = "1506";
+          };
+        };
+        symlink = "/dev/modem";
       })
     ];
   };

modules/service-trigger/default.nix

@@ -0,0 +1,37 @@
+# this is unlikely to be the final form or location of this code, it's
+# an interim module which wraps the uevent-watch command
+
+{ lib, pkgs, config, ... }:
+let
+  inherit (lib) mkOption types;
+  inherit (pkgs) liminix;
+#  inherit (pkgs.liminix.services) bundle;
+in {
+  options = {
+    system.service.uevent-rule = mkOption {
+      description = "a service which starts other services based on device state (sysfs)";
+      type = liminix.lib.types.serviceDefn;
+    };
+  };
+  config = {
+    system.service.uevent-rule = liminix.callService ./rule.nix {
+      service = mkOption {
+        description = "the service to run when the rule matches";
+        type = liminix.lib.types.service;
+      };
+      terms = mkOption {
+        type = types.attrs;
+        example = {
+          devtype = "usb_device";
+          attrs.idVendor = "8086";
+        };
+        default = {};
+      };
+      symlink = mkOption {
+        description = "create symlink targeted on devpath";
+        type = types.nullOr types.str;
+        default = null;
+      };
+    };
+  };
+}

modules/service-trigger/rule.nix

@@ -0,0 +1,25 @@
+{
+  liminix
+, uevent-watch
+, serviceFns
+, lib }:
+{
+  service, terms, symlink
+}:
+let
+  inherit (liminix.services) longrun;
+  inherit (lib.attrsets) collect mapAttrsRecursive;
+  inherit (lib.strings) concatStringsSep;
+  stringify = attrs :
+    concatStringsSep " "
+      (collect lib.isString
+        (mapAttrsRecursive
+          (path : value : "${concatStringsSep "." path}=${value}")
+          attrs));
+  termsString = stringify terms;
+in longrun {
+  name = "watch-for-${service.name}";
+  isTrigger = true;
+  buildInputs = [ service ];
+  run = "${uevent-watch}/bin/uevent-watch ${if symlink != null then "-n ${symlink}" else ""} -s ${service.name} ${termsString}";
+}